In some computer architectures the use of very long instructions words, such as instructions words which are 60-bits or 64-bits long, can facilitate efficient performance. However, the use of long instruction words is expensive in terms of program storage because many program instructions do not require the use of such long instructions words.
In digital signal processors, program instructions can generally be divided into load/store instructions, which load values from the processor's random access memory into the processor's registers or store back register values into memory, and arithmetic instructions, which perform mathematical computations. While the mathematical computations make use of the processor's registers, the digital signal processor of the present invention takes advantage of the fact that a load/store instruction can be performed simultaneously with an arithmetic instruction.
In practice, concurrent execution of load/store and arithmetic operations is use primarily in the "number-crunching" portions of digital signal processing programs. Almost all other portions of digital signal processing programs comprise sequences of load/store operations or sequences of arithmetic operations which cannot be executed concurrently. Therefore it is not efficient to store in program memory long instruction words which contain both load/store and arithmetic operations.
The present invention forms long instruction words from sequences of shorter instruction words when combining such sequences of shorting instruction words into a single instruction comprises an allowed combination, as determined by a set of predefined rules. Short instructions each perform only a load/store or only an arithmetic operation. A long instructions is a combination of a load/store instruction and an arithmetic instruction. Long instructions can also be applied to implement two-word instructions in one machine cycle. The present invention allows the system to store short instruction words in external program memory, which is efficient for memory storage purposes, and to dynamically combine short instructions into long instructions, which is efficient for executing programs.